A microemulsion is a liquid mixture comprising at least three components of water phase (aqueous solution), oil phase (organic solvent) and surfactant, it is a transparent or semi-transparent, thermodynamically stable dispersing liquid with macro-homogeneity and micro-heterogeneity, having the characteristics of low viscosity and isotropy etc. In a microemulsion, continuous media are dispersed into numerous micro-spaces which have small, uniform particle sizes and high stability. Generally, a microemulsion can exist stably for several months. There are three types of microemulsions, i.e. water-in-oil microemulsions (W/O), oil-in-water microemulsions (O/W) and bicontinuous microemulsions.
Microemulsions are widely used and play an important role in the fields including petroleum, cosmetics, medicine, polymers, spinning, papermaking and printing; and especially widely used in the fine chemistry industry such as pesticides, pharmaceutics, cosmetics, cooling fluids for metal, liquid detergents, auxiliaries for oil field exploitation, printing and dyeing auxiliaries.
The formation of a microemulsion mainly depends on the matching of each component in the system. The factors determining the stability of a microemulsion mainly are the kinds of the surfactants, and the ratio of surfactant to oil or water. There are many kinds of surfactants generally used in preparing a microemulsion, including anionic surfactant, cationic surfactant and non-ionic surfactant.
As to conventional surfactants such as perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) or fluoropolyether, since their emulsifying capability can only reach an average particle size range of 150-300 nm in dispersion polymerization reaction and emulsion polymerization reaction, the formed micelles will be destroyed by violently stirring, and breaking emulsion too early in the polymerization process will bring unsafe explosive polymerization to the polymerization reaction of tetrafluoroethylene etc.